The present invention relates to a movement detector pad which comprises a pair of plates between which a piezoelectric compression transducer is secured and more specifically to resilient attachment members which interconnect the plates together and provide attachment and resilient displacement of the plates with respect to one another.
Reference is made to U.S. Pat. No. 6,146,332 which discloses a motion detector of the general type as described herein. In that patent the collector and backing plates are molded from plastic material and interconnected together by connector posts which are rigidly secured in the corner of the plates at distances which are remote from the piezoelectric compression transducer. Therefore, the plates in these corner areas are fairly rigid and will shunt any force applied to these corner areas due to its rigidity thereby often not detecting movement over these corner areas.
In that patent it is also disclosed that to maintain the plates substantially in parallel planes, due to the fact that support posts are provided in the corners only, resilient supports are provided at other locations and particularly adjacent the side edges of the plates in alignment with the central transducer. These resilient supports are helical compression springs which are retained captive within opposed recesses formed in the collector plate as well as the backing plate. These compression springs have a selected compression strength sufficient to maintain the plates spaced apart, in the area of the transducer, and permitting the plates to flex towards one another for sensing the displacement of a load positioned above a mattress or any other medium disposed on top of a detector. In particular, this movement detector is to detect child movement over a mattress of a crib. These helical compression springs do not resolve the above-mentioned problem of stiffness in the corner areas of the plates shunting movement thereover.
It is a feature of the present invention to provide a movement detector which substantially overcomes the above-mentioned disadvantages of the prior art.
According to the above feature, from a broad aspect, the present invention provides a movement detector comprising a pair of flat plates supported spaced apart in juxtaposed facial alignment and disposed substantially parallel relationship by resilient attachment members which are secured between the plates at predetermined positions adjacent an outer circumferential edge of the plates. A piezoelectric compression transducer is secured between the plates substantially centrally thereof and generates electric signals when the plates are displaced with respect to one another by displacement of an exterior load positioned above a top one of the plates. The top one of the plates is a collector plate and the other plate is a backing plate. The resilient attachment members spring bias the plates apart and interconnect the plates in the corner areas of the plates.
According to a further broad aspect, the attachment members each have a connector pin connected to one of the plates and extending transversely to an inner surface of one of the plates. The connector pin has lateral projecting means spaced a predetermined distance from the inner surface. A pin receiving hollow node projects above an inner surface of the other of the plates and aligned with the connector pin. The hollow node has a flat top wall with a hole therein for receiving the pin therethrough. A ledge is formed about the hole by the top wall. The pin extends into the hole with the lateral projecting means captive thereunder and disposed for facial contact with an inner face of the ledge when biased thereagainst. A helical spring is retained in position about the connector pin and the pin receiving node and between said plates to spring bias the plates apart with the plates being connected together by the connector pin captive in the node with the lateral projecting means spring biased against the ledge.
According to a still further broad aspect of the present invention the resilient attachment members are constituted by leaf springs interconnecting corner portions of the pair of plates and biasing the plates spaced apart in substantially facially aligned spaced parallel relationship.